Method for producing a thermoplastic film

ABSTRACT

In a method for producing a thermoplastic film having a three-dimensionally textured, embossed surface, the film is subjected prior to a subsequent shaping kind of processing step to electron-beam crosslinking, which differently crosslinks the individual surface-area regions of the film, so that the regions that undergo greater degrees of drawing out during deforming have different degrees of crosslinking than their neighboring regions.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copendinginternational application No. PCT/EP2007/050325, filed Jan. 15, 2007,which designated the United States; this application also claims thepriority, under 35 U.S.C. §119, of German patent application No. DE 102006 011 159.1, filed Mar. 10, 2006; the prior applications are herewithincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for producing a thermoplastic filmhaving a three-dimensionally structured surface, the film alreadyprovided with a surface structure in a preceding shaping step beingsubjected to a further shape-imparting processing step, in particularshape-imparting thermoforming, in which the film acquires its componentform. The invention also relates to a film for a processing stepincluding shaping.

The preceding shaping step in which the film is provided with a surfacestructure contains as a rule an embossing method. Familiar as thesubsequent shape-imparting processing step is not only thermoforming butof course a number of further forming methods, such as, for example,pressure or pressing methods in which the film is pressed against moldsand acquires its component form.

Thermoplastic film having a three-dimensionally structured, embossedsurface, i.e. for example grained, patterned or finely structured moldedplastic skins, are widely known as surfaces for articles and are used,for example, for the interior cladding in vehicles, here often asrelatively flexible foam-backed films, so-called foam films, havingpleasant haptic properties, for example for the cladding of dashboardsor the interior shells of doors, etc. With appropriate adaptation ofstrength and design, such films are of course also used for other goodsprovided with a high-quality coating.

The prior art discloses various methods for producing such molded skins,for example rolling methods for producing “continuous” film webs ormethods for producing individual molded skins directly from the mold.Here, however, the rolling methods in which a thermoplastic film isprovided with a surface structure with the aid of an embossing roll arediscussed below.

In relation to the following deformation processes, the person skilledin the art is familiar with the problem that, on application of a filmto a three-dimensional component, i.e. for example in a drawing processin which a film provided with a uniformly embossed surface is drawn intoa mold (thermoforming) or is clamped over a fixed support or base body,deformations of the film can of course occur which go beyond the limitof elasticity of the material of which the work piece consists.Distortions may arise as a result of changing distances between theindividual surface regions, which are immediately evident to the vieweras irregularities. Since a strong trend toward improving the qualityimpression is to be observed in the region of the interior ofautomobiles, such irregularities are less and less acceptable.

In this context, German patent DE 102 02 752, corresponding to U.S. Pat.No. 6,913,728, discloses a process for the production of a shapedarticle which is thermoformed from a thermoplastic film and in which thesurface structure of the embossing roll is densified or reduced in sizein the regions in which an expansion of the thermoplastic film takesplace during the thermoforming process. This compensation then resultsin a uniform surface pattern during thermoforming. Here, however, theembossing roll or the outer roll surface serving as a negative must beprocessed in the form of a silicone tube in a relatively complicatedmanner in order to establish the compensating pattern densifications.

Published, non-prosecuted German patent application DE 100 18 196 A1,corresponding to U.S. Pat. No. 6,663,738, describes a process for theproduction of a grained film from uncrosslinked polyolefins, which istreated with electron beams to increase the grain tightness and is thenthermoformed. Since the film as a whole has a more stable and hence lessdeformable grain as a result of such a process, only the elongation as awhole is reduced but the problem of the required different elongation ofindividual regions of the film is not satisfactorily solved.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method forproducing a thermoplastic film which overcomes the above-mentioneddisadvantages of the prior art methods of this general type, and whichpresents an economical process by which a film suitable for subsequentshape-imparting processing, in particular for thermoforming, can beproduced, which process permits different deformations/elongations overthe individual surface regions of the film without permitting visibledistortions due to changing distances between individual surfacestructures to be recognizable.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a process for crafting a thermoplasticfilm having a three-dimensionally structured surface. The processincludes the steps of providing a film having a surface structure formedin a preceding shaping step; subjecting the film to electron beamcrosslinking for crosslinking individual regions of the film differentlyand substantially in a manner such that regions subjected to higherdegrees of drawing in a subsequent shape-imparting processing step havedegrees of crosslinking differing from neighboring regions; andsubsequently subjecting the film to a shape-imparting processing step.

Here, the film, which as a rule was extruded, already embossed and, ifappropriate, also already coated, is subjected before the subsequentshape-imparting processing step to electron beam crosslinking whichcrosslinks the individual extensive regions of the film differently andsubstantially in a manner such that the regions which are subjected tohigher degrees of drawing in the subsequent shape-imparting processingstep have degrees of crosslinking differing from their neighboringregions.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is described herein as embodied in a method forproducing a thermoplastic film, it is nevertheless not intended to belimited to the details described, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The crosslinking of polymers takes place through formation of covalentbonds between the polymer chains. Usually, the crosslinking is effectedby the classical vulcanization with elemental sulfur or silanes, byperoxide crosslinking, by crosslinking with electron beams or by acombination of the methods. In electron beam crosslinking, the freeradicals initiating the crosslinking process form through the action ofhigh-energy radiation on the polymer molecules. The acceleratedelectrons interact with the irradiated molecules. The kinetic energy ofthe electrons is transferred to atoms with the molecular assembly byelastic impacts. The atoms affected are thus converted into a morehighly excited state. If the energy thus supplied is greater than thebond energy of the covalent bonds, the bond is cleaved and freeradicals, macroradicals and ions form.

The free radicals react in subsequent reactions with the molecules ofthe polymer chains or with themselves and lead both to the formation ofcovalent bonds between the individual chains and to the degradation ofthe macromolecules by chain cleavage. Chain cleavage and chain synthesistake place in parallel. The polymer type used and the processingconditions, such as radiation dose, type of radiation, temperature, etc.determine which reaction dominates. Establishing process parameters inelectron beam crosslinking is therefore of primary importance. Sincechain degradation also takes place, crosslinking of all polymer chainspresent with one another cannot be realized. Although completecrosslinking is not achieved by radiation crosslinking, main features ofthe irradiated polymers are nevertheless strongly influenced.

The crosslinking points newly formed during crosslinking induced byelectron beams hinder the folding process of the polymer chains. Thisresults in a reduction in the crystallinity, in particular themechanical strength and brittleness decreasing with decreasingcrystallinity whereas toughness and insulation properties increase. Inmost cases, however, a decrease in the strength which is to be expectedowing to the decrease in crystallinity does not in fact occur. Thereason for this is that the lower crystallinity is overcompensated bythe increased structural strength of the crosslinked amorphous regions.The cohesive forces between the crosslinked polymer chain segments areseveral times greater than in the uncrosslinked state, where only vander Waals interaction forces act between the chains. The sliding anddisplacement of the polymer chains are made substantially more difficultas a result of the crosslinking. These changes manifest themselves, forexample, in an increase in the mechanical strength and the heatdistortion resistance.

Because the regions which are subjected to higher degrees of drawing inthe subsequent shape-imparting processing step have degrees ofcrosslinking differing from their neighboring regions, in particular andadvantageously higher degrees of crosslinking, an extremely uniformsurface structure is retained even after the subsequent deformationstep, for example after the application of the film to athree-dimensional component.

This is because the strongly formed film regions, i.e. for example thoseon projecting geometries having small radii, extend to a greater degreethan the neighboring regions and therefore transmit the deformationstresses to the neighboring region. Viewed over the total area, thesurface tension of the film after the deformation then becomes uniform.As a result, the surface structure is also substantially retained, butin any case so that a change is not noticeable to the naked eye. In theconventional processes in the prior art, the regions having higherdegrees of drawing had to adsorb the total elongation alone, i.e. forexample were elongated by 60%, while the directly adjacent regions werenot elongated. The differences in the structure were therefore oftennoticeably large. In the process according to the invention, theelongation of the regions having higher degrees of drawing is greatlyreduced by the crosslinking, with the result that transmission of theforming stresses also takes place to the adjacent regions so that bothregions are elongated by approximately equal amounts of, for example25-30%. Structural differences in the transition between the regions arethus substantially reduced.

A distribution of the newly formed crosslinking points which isinhomogeneous over the film surface, i.e. the crosslinking densitydistribution or network arc density—for example expressed by the gelcontent as a known measure for the crosslinking—can be particularlyadvantageously achieved if the film is exposed from both sides toelectron radiation, it being possible for the degrees of crosslinking tobe different on the two film sides or film surfaces.

As a result, the total crosslinking of individual regions can beinfluenced both via the irradiated area and via the radiation intensityand thereby influenceable effect of the crosslinking in the thicknessdirection of the film.

In the case of irradiation on one side, the region of maximum doseadsorption can be varied and hence defined in the case of thecrosslinking density distribution by the choice of the accelerationvoltage for the electrons, depending on the film thickness.

In the case of irradiation on two sides, the crosslinking densitydistribution can moreover be influenced by the relation of the applieddoses with variation of the respective acceleration voltage. Dependingon the respective compositions of the films to be irradiated, these haveto be adapted again for each chemical system.

In an advantageous further development, the electron beam crosslinkingof the film is effected by irradiation of at least one film surface withan electron beam source several times at least in regions. The desireddifference in the crosslinking can be produced simply through the simplelocal control of the radiation source of the electron beam. Since thefilms to be processed are usually present as web material, the width ofthe film webs being determined by the preceding production, it is ofcourse possible in the context of the invention to carry out differentirradiation variants which are adapted to the further processing of thefilm web. If, for example, it is certain that the middle region of afilm web is always that which occurs in the region of the instrumentcover of a dashboard during the subsequent shaping and hence masked bearthe greatest degrees of drawing, it is just this middle region of thefilm which is crosslinked once or several times according to theinvention.

An advantageous further development relates in carrying out the electronbeam crosslinking of the film surface line by line in succession, thebeam width of the electron beam being adjustable by use of an aperture.With the aid of such “scanning”, a degree of crosslinking differing overthe film width can be established in a simple manner.

This also applies to another advantageous further development whichrelates to arranging a mask, which changes the intensity of the electronradiation at least in partial regions of the beam cross section, betweenthe electron beam source and the irradiated film surface.

A thermoplastic film of crosslinked polymeric material having athree-dimensionally structured, embossed surface which is formed of athermoplastic elastomer, in particular a thermoplastic olefin (TPO) or apolyolefin mixture, is particularly suitable for use in the processaccording to the invention.

The particular advantage of using this polymer type in the case of thefilm according to the invention is that the originally presentintermolecular crosslinking of a thermoplastic olefin (hydrogen bridges,crystalline structures) is predominantly thermoreversible andsubstantially of a physical nature, which is fundamental with regard tothe suitability for deformation. The “additional” electron beamcrosslinking of certain regions of the polyolefin provides theparticular and surprising property of the film in which on the one handelongation behavior required for the forming and on the other handsufficient resistance to excessive elongations of the surface forreliable material handling of the process are present during the formingstep usually taking place at elevated temperature.

In another advantageous further development, the film formed ofprecrosslinked polymeric materials, in particular of a composition ofpolypropylene, polyethylene and copolymers and terpolymers thereof,which are particularly suitable for use as a film for a motor vehicleinterior. This too gives a particularly uniform surface structurewithout striking excess elongations after the subsequent deformationstep. The precrosslinking is effected by chemical method by addition ofcustomary crosslinking agents.

In another advantageous further development, the film is in the form ofa multilayer polymer film composite. Such a formation promotes theinfluenceable effect of the crosslinking in the thickness direction ofthe film and hence the total crosslinking of individual film regions.

The constituents of the polymer films are preferably polyolefins. Therange of polyolefins which may be used is not subject to any fundamentallimitation. Polyolefins, such as PP, PE, poly(1-butene),polyisobutylene, poly(4-methylpentene), PP copolymer or terpolymers withC₂, C₄-C₁₂-α-olefins, PE copolymers or terpolymers with C₃ toC₁₂-α-olefins or mixtures thereof can preferably be used, it also beingpossible to use as co- or termonomers diene monomers which containnonconjugated double bonds, such as, for example, 1,4-hexadiene,5-methyl-1,5-hexadiene, 5-ethylidene-2-norbornene,5-butylidene-2-norbornene, dicyclopentadiene, 1,4-octadiene,cyclohexadiene or cyclooctadiene; copolymers of propylene and/orethylene with polar comonomers, such as acrylic acid and/or theC₁-C₁₂-esters thereof, methacrylic acid and/or the C₁-C₁₂-estersthereof, ionomers based on acrylic acid and/or with methacrylic acid andsulfuric acid, vinyl esters of saturated C₁-C₈-carboxylic acids,optionally with carbon monoxide as a termonomer; graft copolymers ofpropylene and/or ethylene having 8-45% of grafted-on units ofunsaturated carboxylic acids, dicarboxylic acids, the esters and/oranhydrides thereof and mixtures of the polymers.

In another advantageous further development, the film has a thickness offrom 0.4 to 4 mm. As a result, the adjustability of the depth ofcrosslinking is further facilitated.

With the aim of making the elongation highly uniform, the crosslinkingof the film is advantageously adjusted so that, in the regions which aresubjected to higher degrees of drawing in the subsequent shape-impartingprocessing step, the film has a gel content of at least 30%, preferablya gel content of from 40 to 60%. As a result, the grain tightness of theelongated regions of the film is sufficiently high to prevent adistortion of the surface structure/grain structure, the other regionsof the film which have the lower gel content providing sufficientextensibility to achieve reliable deformation in the process forcovering a three-dimensional component.

In an advantageous development, the difference in the gel contentbetween regions of the film which have a high degree of crosslinking anda low degree of crosslinking is from 10 to 60%, preferably from 20 to50%. As a result, sufficient uniformity of the elongations of thematerial is achieved even in the case of strongly deformed components,such as, for example, in the case of covers for the transmission tunnelof a car.

The determination of the gel content is usually effected via anextraction method in which first samples with a thickness of about 0.5mm are cut into squares having an edge length of about 1.0 mm. Thesamples (about 100 mg) are then initially introduced into test tubeswhich are provided with plugs which are made of stainless steel wire andprevent the samples from floating. The test tubes are filled with 100 mlof xylene and closed with aluminum foil in order to prevent evaporationof the solvents. The xylene is then heated to the boil. The testspecimens are left in the boiling xylene for about 24 h. Thereafter, thegel-xylene mixture is filtered over a drum screen having a mesh size of200 mesh, the gel remaining in the drum screen. The drum screens areplaced on metal plates and dried at 140° C. for 3 h in athrough-circulation oven. After cooling to room temperature, the contentis weighed out and related to the sample weight.

The abovementioned production process can be particularly advantageouslyused for a dashboard for the interior cladding of motor vehicles with anouter surface in the form of a foam-backed film. Such dashboards oftenhave highly formed regions which are directly and permanently visible todriver and passenger. This applies, for example, to the instrumentcover, to the glove compartment and to ventilation nozzles and cut-outs.Here, making the elongations uniform, as is achieved in the film by theprocess according to the invention, is particularly important foresthetic reasons.

The invention claimed is:
 1. A thermoplastic film, comprising:crosslinked polymeric materials having a three-dimensionally structured,embossed surface; the crosslinked polymeric materials including firstregions and second regions, said first regions being subjected to higherdegrees of drawing in a subsequent shape-imparting processing step thansaid second regions; said crosslinked polymeric materials being formedby subjecting a film to electron beam crosslinking for crosslinking saidfirst regions differently from said second regions, such that said firstregions have degrees of crosslinking higher than neighboring secondregions and said first regions have a greater strength than neighboringsecond regions; said film having a gel content, said first regionshaving a gel content of at least 30%, and a difference in said gelcontent between said first regions of said film having a high degree ofcrosslinking and said second regions of said film having a low degree ofcrosslinking being from 10 to 60%; and said film formed from athermoplastic elastomer.
 2. The thermoplastic film according to claim 1,wherein said crosslinked polymeric materials formed of a composition ofpolypropylene, polyethylene and copolymers and terpolymers thereof. 3.The thermoplastic film according to claim 1, wherein the thermoplasticfilm is a multilayer polymer film composite.
 4. The thermoplastic filmaccording to claim 1, wherein the thermoplastic film has a thickness offrom 0.4 to 4 mm.
 5. The thermoplastic film according to claim 1,wherein said thermoplastic elastomer is selected from the groupconsisting of a thermoplastic olefin and a polyolefin mixture.
 6. Thethermoplastic film according to claim 1, wherein said crosslinkedpolymeric materials formed of a composition of polypropylene,polyethylene and copolymers and terpolymers thereof are for use as afilm for a motor vehicle interior.
 7. A dashboard for interior claddingof vehicles, the dashboard comprising: an outer surface formed from afoam-backed thermoplastic film according to claim 1.